Cyclins and their related proteins in pituitary tumourigenesis.

Pituitary tumours are benign neoplasms that may cause major endocrine dysfunction. Transgenic disruption of the cell cycle machinery frequently leads to pituitary adenoma formation in animal models. The molecular analysis of human pituitary tumours has found various alterations in the expression of cell cycle regulators: cyclins, cyclin-dependent kinases and their inhibitors. There are also different mechanisms (e.g. hypermethylation, frameshift mutations, increased proteasome degradation) responsible for changed expression in cyclin mRNA and protein. It is probable that the primary initiating events lie beyond the cell cycle and may be related to co-activation of Akt, MAP-kinase and beta-catenin pathways. Nevertheless, molecular CDK inhibitors may play a role in pituitary tumour treatment in the future.

Differential gene expression in pituitary adenomas by oligonucleotide array analysis.

OBJECTIVES: Microarray technology allows for the expression profile of many thousands of genes to be quantified at the same time, and has resulted in novel discoveries about the tumour biology of a number of cancers. We sought to do this in pituitary adenomas, the most common intracranial neoplasm. METHODS: Affymetrix GeneChip HG-U133A oligonucleotide arrays covering 14 500 well-characterised genes from the human genome were used to study pooled RNA for each of the four major pituitary adenoma subtypes. Individual gene-expression levels in the tumours were compared relative to the expression profile in normal pooled pituitary RNA. Three differentially expressed genes with potential importance in tumourigenesis were chosen for validation by real-time quantitative PCR on the original tumours and on an additional 26 adenomas. RESULTS: Bioinformatic analysis showed that 3906 genes and 351 expressed sequence tags were differentially expressed among all pituitary tumour subtypes. Lysosomal-associated protein transmembrane- 4-beta (LAPTM4B), a novel gene upregulated in hepatocellular carcinoma, was significantly over-expressed in adrenocorticotrophin (ACTH)-secreting adenomas and non-functioning pituitary adenomas (NFPAs). Bcl-2-associated athanogene (BAG1), an anti-apoptotic protein found at high levels in a number of human cancers, was significantly over-expressed in growth hormone-secreting and prolactin-secreting adenomas and NFPAs. The cyclin-dependent kinase inhibitor p18, in which murine gene deletion has been shown to produce pituitary ACTH cell hyperplasia and adenomas, was significantly under-expressed in ACTH-secreting adenomas. CONCLUSIONS: Expression array analysis of pituitary adenomas using the Affymetrix GeneChip HG-U133A arrays appears to be a valid method of identifying genes that may be important in tumour pathogenesis.

Role of regulatory factors in pituitary tumour formation.

The molecular basis of pituitary tumorigenesis remains controversial, but there are two major theories which have been subject to most investigation: hormonal (usually hypothalamic factors) and/or growth factor overstimulation, or a molecular defect within the pituitary itself. It has been shown, for example, that excessive regulatory hormone stimulation can lead to an increased number of cells in the pituitary in various physiological or pathological states such as pregnancy (lactotrophs), untreated primary hypothyroidism (thyrotrophs and lactotrophs),primary hypoadrenalism (corticotrophs) and ectopic GHRH-secreting tumours (somatotrophs). Animal models also provide data that in the presence of excessive hypothalamic hormone stimulation, adenoma formation can occur. However, evidence in favour of the monoclonal nature of pituitary tumours argues for an intrinsic molecular defect as the primary initiating event in tumour formation. We review the various hormonal factors and their receptors effecting the different types of pituitary cells, such as CRH, AVP and cortisol feedback on corticotrophs; GHRH, Galpha PKA, somatostatin and GH and IGF feedback on somatotrophs; GnRH, LH/FSH, activin and oestrogen feedback on gonadotrophs; dopamine, oestrogen and prolactin feedback on lactotrophs; and TRH, TSH and thyroid hormone feedback on thyrotrophs. The monoclonal origin of adenomas makes it unlikely that hypothalamic factors could initiate pituitary transformation, but they could still create an environment where there is a higher chance that a possible causative tumorigenic mutation may occur in one (or several) of the overstimulated pituitary cells, or enhance the proliferation of an already-mutated cell.

Identification of adrenocorticotropin receptor messenger ribonucleic acid in the human pituitary and its loss of expression in pituitary adenomas.

The ACTH receptor (ACTH-R) is the second member of the melanocortin (MC-2) receptor family that includes five seven-transmembrane G protein-coupled receptors and has been shown to be predominantly expressed in the adrenal cortex. It has been postulated that ACTH may regulate its own secretion through ultra-short-loop feedback within the pituitary. ACTH-secreting adenomas are characterized by resistance to glucocorticoid feedback, and they may have dysregulated ACTH feedback. We therefore investigated the ACTH-R in normal and adenomatous human pituitary tissue. We report here the identification of ACTH-R mRNA in the human pituitary gland, which was confirmed by direct sequencing. We studied the expression of the ACTH-R in 23 normal pituitary specimens and 53 pituitary adenomas (22 ACTH-secreting, nine GH-secreting, eight prolactin-secreting, one TSH-secreting, one FSH-secreting, 10 nonfunctioning, and two silent corticotroph adenomas), using the sensitive technique of real-time quantitative PCR. Contamination of ACTH-secreting adenomas and nonfunctioning pituitary adenomas with nonadenomatous tissue was excluded by lack of Pit-1 expression. ACTH-R mRNA was detected in all normal pituitary specimens, and in situ hybridization colocalized expression to ACTH staining cells only. However, ACTH-R mRNA levels were undetectable in 16 of 22 ACTH-secreting tumors and in both silent corticotroph tumors. Diagnostic preoperative plasma ACTH levels were significantly lower in the ACTH-R positive ACTH-secreting tumors, compared with those who were ACTH-R negative (P = 0.0006). Direct sequencing of the coding region of the ACTH-R in cDNA from three ACTH-secreting tumors positively expressing the receptor showed no mutations, as did sequencing of genomic DNA in three receptor negative ACTH-secreting tumors and the two silent corticotrophs. These results provide further evidence compatible with an ACTH feedback loop in the pituitary and suggest that loss of expression of the ACTH-R in corticotroph adenomas of patients with Cushing's disease may play a role in the resistance to feedback of the pituitary-adrenal axis seen in these patients.

Discriminatory value of the low-dose dexamethasone suppression test in establishing the diagnosis and differential diagnosis of Cushing's syndrome.

Cushing's syndrome requires a screening test of high sensitivity, followed by biochemical evaluation of the source of the tumor when the cause is ACTH dependent. The high-dose dexamethasone suppression test is still in common use as an aid in differential diagnosis, although its value has been queried. We have routinely used the low-dose dexamethasone suppression test for many years in the diagnosis of Cushing's syndrome but noticed that patients with pituitary-dependent Cushing's syndrome or Cushing's disease, usually showed some degree of suppression of their serum cortisol, compared to those with the ectopic ACTH syndrome. We therefore analyzed retrospectively the serum cortisol responses during the low-dose dexamethasone suppression test and the high-dose dexamethasone suppression test in 245 patients with ACTH-dependent Cushing's syndrome and compared the diagnostic utility of each test either alone or in combination with a standard test using CRH. Evaluation of the serum cortisol response at 24 and 48 h during the low-dose dexamethasone suppression test correctly identified 98% of patients with ACTH-dependent Cushing's syndrome and distinguished between pituitary and ectopic causes with a sensitivity of 82% and a specificity of 79%. In the same patients, the serum cortisol response to the high-dose dexamethasone suppression test had a slightly higher sensitivity (91%) and specificity (80%). However, the combined criteria of a more than 30% suppression of serum cortisol during the low-dose dexamethasone suppression test and/or a more than 20% increase in the CRH test had a significantly higher sensitivity (97%) and specificity (94%) than either the high-dose dexamethasone or the CRH tests alone in the differential diagnosis of ACTH-dependent Cushing's syndrome. It produced equivalent information to that when high-dose and CRH test results were combined. We therefore conclude that in our patient series, the serum cortisol response during the low-dose dexamethasone suppression test is highly sensitive in diagnosing Cushing's syndrome and, combined with the results of the serum cortisol response to the CRH test, offered a safe and cost-effective test in the differential diagnosis of ACTH-dependent Cushing's syndrome. There does not appear to be any necessity for retaining the high-dose dexamethasone suppression test in this diagnostic work-up.

Reduced expression of the growth hormone and type 1 insulin-like growth factor receptors in human somatotroph tumours and an analysis of possible mutations of the growth hormone receptor.

OBJECTIVE: Clinical acromegaly is characterized by elevated GH secretion in the presence of high circulating IGF-I levels. We hypothesized that the physiological IGF-I/GH negative feedback loop may be reset in somatotroph adenomas, specifically in terms of the level of expression of these receptors or mutations of the GH receptor (GH-R) in such tumours. METHODS: We therefore investigated the full coding sequence of the GH-R in a series of somatotroph and other pituitary adenomas. We also investigated the mRNA expression of these putative feedback receptors in a series of pituitary adenomas and normal pituitary tissue, and their protein expression by immunostaining. Real-time RT-PCR assay was used for the quantification of the type 1 IGF receptor (IGF-R) and GH receptor (GH-R) mRNA, and sequence analysis was performed on the coding region of the GH-R gene. RESULTS: No somatic mutations of the GH-R mRNA were detected in 18 GH-secreting tumours or two nonfunctioning pituitary adenomas (NFPAs). However, the levels of GH-R mRNA were significantly lower in both somatotroph tumours and NFPAs compared to the normal pituitary (P < 0.05 for both). Immunostaining for GH-R also showed significantly less GH-R expression in somatotroph adenomas compared to normal pituitary tissue (P < 0.0001). IGF-R mRNA levels were significantly lower in somatotroph tumours compared to normal pituitary (P = 0.005), and trended lower in corticotroph tumours (P = 0.07), while the other tumour types showed no significant difference from normal pituitary. Immunostaining for IGF-R also showed less IGF-R protein in the somatotroph adenomas compared to the normal pituitary tissue (P < 0.01). CONCLUSIONS: Our findings suggest that decreased feedback inhibition of GH because of somatic mutations of the coding region of the GH-R are unlikely to be a common factor in the pathogenesis of these tumours. Nevertheless, decreased expression of the GH-R and of IGF-R in somatotroph tumours (both at the mRNA and protein level) may, at least in part, help explain the continuous secretion of GH from the tumour despite the high circulating levels of IGF-I and GH.

Sequence analysis of the PRKAR1A gene in sporadic somatotroph and other pituitary tumours.

OBJECTIVE: Carney complex (CNC) is an autosomal dominant multiple neoplasia syndrome featuring cardiac, endocrine, cutaneous and neural tumours, as well as a variety of pigmented lesions of the skin and mucosa. Pituitary GH-secreting tumours are found in approximately 10% of patients with CNC. One of the genes responsible for CNC, the PRKAR1A gene located on human chromosome 17q22-24, has recently been cloned. This represents a putative tumour suppressor gene, coding for the type 1alpha regulatory subunit of protein kinase A (PKA), which is found to be mutated in approximately half of the patients with CNC. However, it is currently unclear as to whether similar mutations occur in sporadic pituitary tumours. We have therefore investigated a series of GH-secreting and other pituitary tumours for sequence abnormalities in the PRKAR1A gene. The mRNA produced by the PRKAR1A undergoes decay if it codes for a truncated protein; we therefore also determined PRKAR1A mRNA levels in the tumours, and compared them with known mutant PRKAR1A-carrying lymphocyte samples. METHODS: We extracted RNA from a series of pituitary tumours, reverse transcribed it to cDNA, and directly sequenced the PRKAR1A coding sequence in 17 GH-secreting, three prolactin-secreting, three ACTH-secreting, one FSH-secreting and 10 nonfunctioning pituitary tumours. Lymphocyte and tumour tissue RNA from two patients with CNC was used as positive controls. Using duplex polymerase chain reaction (PCR) with the PRKAR1A and the "housekeeping" gene GAPDH, we determined the relative expression of the PRKAR1A gene in the unknown as well as in the positive control samples. RESULTS AND CONCLUSION: No mutations were found in any of the exons sequenced. Relative mRNA expression was not decreased in any of the sporadic pituitary tumour samples. The present data thus do not suggest a major role for the PRKAR1A tumour suppressor gene in sporadic GH-secreting or other pituitary tumours.

The tissue distribution of the mRNA of ghrelin and subtypes of its receptor, GHS-R, in humans.

Ghrelin is a novel growth hormone-releasing peptide, originally identified in the rat stomach as the endogenous ligand for the growth hormone secretagogue-receptor (GHS-R1a). Ghrelin is involved in the regulation of GH release, but it has recently been suggested that ghrelin may have other actions, including effects on appetite, carbohydrate metabolism, heart, kidney, pancreas, gonads, and cell proliferation. The distribution of ghrelin, its functional receptor (type 1a) and the unspliced, non-functional GHS-R type 1b mRNA expression was investigated in various human tissues using classical and real-time reverse transcription and polymerase chain reaction. GHS-R1a was predominantly expressed in the pituitary and at much lower levels in the thyroid gland, pancreas, spleen, myocardium and adrenal gland. In contrast, ghrelin was found in the stomach, other parts of the gut and, indeed, in all the tissues studied (adrenal gland, atrium, breast, buccal mucosa, esophagus, Fallopian tube, fat tissue, gall bladder, human lymphocytes, ileum, kidney, left colon, liver, lung, lymph node, muscle, muscle, myocardium, ovary, pancreas, pituitary, placenta, prostate, right colon, skin, spleen, testis, thyroid, and vein). GHS-R1b expression was also widespread in all tissues studied. The significance of the widespread tissue distribution of ghrelin remains to be determined. These data suggest that ghrelin might have widespread physiological effects via different, partly unidentified, subtypes of the GHS-R in endocrine and non-endocrine tissues.

The expression of the F-box protein Skp2 is negatively associated with p27 expression in human pituitary tumors.

The CDK inhibitor p27 plays a pivotal role in controlling cell proliferation during development, and has been implicated in tumorigenesis. Previous studies have demonstrated changes in p27 protein expression, but not in mRNA levels, in human pituitary tumors. It seems probable that the fall in p27 is due to increased degradation through the ubiquitin-proteasome pathway. Skp2 (S-phase kinase-interacting protein) is a specific F-box protein that allows the recognition and binding of phosphorylated p27 to the ubiquitin complex. The aim of our study was thus to investigate the possible role of Skp2 in pituitary tumorigenesis. A total of 59 human pituitary samples, 7 normal and 52 adenomas, were assessed for transcriptional expression of Skp2; 51 pituitary samples were assessed for protein expression. Real-time RT-PCR was performed on cDNA of reverse-transcribed mRNA for relative quantification of the Skp2 transcript. Immunostaining was performed using mouse monoclonal anti-Skp2 antibody. Skp2 mRNA and protein was detectable in every sample studied. Our results showed no significant difference between the pituitary tumors and normal pituitary tissue in Skp2 mRNA or nuclear protein expression. Individual tumor types had similar mRNA expression and variable protein expression. However, samples with high p27 protein expression showed significantly less Skp2 expression than samples with low p27 staining. Our data suggest that increased p27 degradation through the ubiquitin-proteasome pathway could be regulated in pituitary tumors by changes in Skp2 expression, although other factors probably also play a role.